The present invention relates generally to imaging spectrometers or "spectrographs" having convex gratings, and more particularly to a compact spectrometer having very low spatial and spectral distortion, optionally combined with a high resolution polychromatic imaging camera.
In many applications, including remote sensing for space and terrestrial exploration, it is desirable to measure incoming radiation in narrow spectral slices from the infrared to the ultraviolet. This has been accomplished using a reflective spectrometer in an "Offner" configuration, i.e., one having a convex diffraction grating in combination with at least one concentric concave mirror, typically operating at unit magnification. Various concentric spectrometer concepts, including the Offner configuration, are disclosed in L. Mertz, "Concentric Spectrographs", Applied Optics, Vol. 16, No. 12 (December 1977), and D. R. Lobb, "Theory of Concentric Designs for Grating Spectrometers", Applied Optics, Vol. 33, No. 13 (May 1, 1994). Such spectrometers can be coupled to fore-optics which are constructed to receive radiation from a target surface and form an image on an entrance slit of the spectrometer. The devices are typically operated in "pushbroom" fashion, during which the device moves in a direction substantially perpendicular to the entrance slit to cover a swath of a target surface. In the course of this operation, white light received through the entrance slit of the spectrometer is diffracted into a continuum of colors and detected by a two-dimensional optical detector for subsequent analysis. Other examples of such systems are disclosed in F. Reininger, "Visible Infrared Mapping Spectrometer-Visible Channel (VIMS-V)," Instrumentation in Astronomy, VIII (March 1994); F. Reininger, "Near Ultraviolet Visible Infrared Mapping Spectrometer (NU-VIMS)," Space Optics, 1994: Earth Observation and Astronomy (April 1994); and Macenka et al. U.S. Pat. No. 5,768,040 for "Wide Field-of-View Imaging Spectrometer."
Unfortunately, prior spectrometers have typically experienced substantial spectral and spatial distortion, or error, resulting largely from the geometry of the overall systems and the resulting mismatch between the fore-optics and the spectrometer portion. In this regard, the detector focal plane and the entrance slit of such spectrometers are typically disposed above and below the diffraction grating, and are displaced from one another in a direction generally parallel to the entrance slit. Although this arrangement has provided reasonably good results, it has not been possible to reduce spatial and spectral distortion ("smile") to the extent desired for accurate spectrographic mapping. In addition, prior spectrometers are not capable of providing high resolution image information because the broad spectrum of incoming light is necessarily broken down into narrow spectra, drastically reducing the signal-to-noise ratio.
Therefore, it is desirable in many applications to provide a spectrometer essentially eliminating spatial and spectral distortion and producing a high quality visual image in conjunction with spectral information.